On Jan 25, 11:59*am, Mark wrote:
* Each time it turns off,

whatever heat remains in the furnace is lost mostly to the
chimney and I would think there would be more of it left in
the old furnace too. *Think heat exchanger mass.

I don't have the answers to your other questions but in my older (oil
fired hot air) furnace with a massive cast iron heat exchanger, *when
the burner shuts off, the air circulation blower remains on until the
heat exchanger is cooled down.

So most of the stored heat is recovered and NOT lost to the chimney.

Mark

It took 5 minutes for my old oil furnace fan would kick on, and vent
temperature would be
as high as 140 degrees, that's after I made the fan go faster. Before
I moved in an uncovered the two main
return vents, I think the furnace would just cycle with overtemp.

I don't have any idea on the OP's situation.

greg